1. Field of the Invention
A number of operations require the temporary installation of access tubes in the soil. For example, the determination of soil water by neutron probe requires that the probe be lowered into soil to various depths within an access tube. Similarly, dewatering wells for excavation sites and hydrologic exploration wells require the installation of tubes that are intended to be extracted later. This invention relates to a versatile device for efficiently and economically extracting soil access tubes while leaving them undamaged and suitable for reuse.
2. Description of the Prior Art
A popular method for making repeated measurements of water in soil involves the use of a neutron probe. The technique is based on the characteristic of small atoms (primarily hydrogen in soils) to deflect and significantly reduce the speed of fast-moving neutrons [Brady, In The Nature and Property of Soils, N. C. Brady (ed.), Macmillan Publishing Co., NY, pp. 164-199 (1974); Hauser, Trans. of the ASAE 27: 722-728 (1984)]. A fast-neutron sealed source (americium-beryllium is commonly used) is contained in the neutron access "probe". For measurement of soil water content, the probe is lowered into the soil profile to various depths within a neutron probe access tube. The soil water content is determined by measuring the relative number of deflected, thermal neutrons that return to the probe as a result of bombardment of the soil by the fast neutrons. The thermal neutron detector is also contained within the probe. The number of deflected neutrons is proportional to soil water content. These devices work best in mineral soils where the vast majority of hydrogen atoms are contained in water molecules.
One of the most important technical considerations for obtaining good readings from neutron probes is correct installation of the access tubes. For best readings, the tubes should fit snugly into the profile where soil cores have been removed. Large air spaces between the access tube and the surrounding soil can cause errors in soil water measurement (Hauser, supra). Removal of correctly installed access tubes, which often are up to 2 m or more in length, can be extremely difficult, especially if the tubes have been in a clay-rich soil for an extended period. Therefore, labor and material costs associated with extracting neutron access tubes can be substantial, especially in heavy soils or with less experienced operators, where tube damage can reach 100% and replacement cost becomes significant.
Several implements for extracting tubes are available, but none are completely satisfactory. One type of extraction device attaches to access tubes by exerting force on the inside of the tube. Such devices can damage access tubes in heavy soils. Furthermore, they cannot be repositioned lower inside tubes during their removal. Thus, such devices have limited capability to extract long tubes with a manual jack, and the result is an awkward and time-consuming operation.